Unexpected vertical black patterns or artifacts such as lines, dots, dashes, etc., have been a common image artifact in binary document images produced by production paper scanners. The vertical black patterns are not part of the original document image, however, the patterns are the result of pieces of dust or dirt blocking light between an illumination light source and the paper documents being scanned. For example, pieces of dust or dirt which come to rest on transparent image guides in an illumination path of the scanner will result in shaded lines upon the document moving through the scanner. When the document is electronically captured by a CCD device, the shaded lines will appear in the digital document image. The shaded vertical lines within a grey scale image will be indistinguishably treated as part of image content. After an adaptive thresholding is applied to the grey scale image, the shaded lines will show up as black lines in the binary image.
One current solution to this problem is to routinely remove the dust or dirt from the illumination path by cleaning the transparent image guides. However, this solution requires interruption of the scanner operation which is undesirable in a production scanner.